首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2750篇
  免费   110篇
  国内免费   15篇
化学   1983篇
晶体学   13篇
力学   80篇
数学   407篇
物理学   392篇
  2023年   24篇
  2022年   49篇
  2021年   61篇
  2020年   69篇
  2019年   56篇
  2018年   66篇
  2017年   59篇
  2016年   111篇
  2015年   97篇
  2014年   117篇
  2013年   203篇
  2012年   226篇
  2011年   223篇
  2010年   141篇
  2009年   116篇
  2008年   183篇
  2007年   175篇
  2006年   127篇
  2005年   122篇
  2004年   133篇
  2003年   89篇
  2002年   68篇
  2001年   27篇
  2000年   25篇
  1999年   22篇
  1998年   13篇
  1997年   10篇
  1996年   22篇
  1995年   11篇
  1994年   13篇
  1993年   15篇
  1992年   15篇
  1991年   10篇
  1990年   14篇
  1989年   12篇
  1988年   8篇
  1987年   15篇
  1985年   11篇
  1984年   14篇
  1983年   5篇
  1982年   9篇
  1981年   8篇
  1980年   12篇
  1979年   7篇
  1978年   7篇
  1977年   6篇
  1976年   7篇
  1975年   9篇
  1973年   5篇
  1971年   5篇
排序方式: 共有2875条查询结果,搜索用时 15 毫秒
41.
Extended labeling experiments have shown that formation of rhodium peroxocarbonate from CO(2) and [RhCl(eta(2)-O(2))(P)(3)] (P is PEt(2)Ph or PEtPh(2)) proceeds through O-O bond cleavage and CO(2) insertion. O-transfer to ancillary phosphine ligand to give R(3)P=O selectively (>85%) involves the Rh-linked O atom of the peroxo group of RhCl(CO(4))(P)(3).  相似文献   
42.
Parchments are important documents that give testimony for History; therefore these materials should be respected and preserved. Considering incremental biodeterioration problems that have to be faced daily, the Archive of the University of Coimbra (AUC) is involved in different scientific projects in order to evaluate and determine new methods for document decontamination and preservation.The aim of this study was to evaluate gamma radiation effects on the colour and texture of the AUC parchment documents. The assessment of these effects was used to estimate the maximum gamma radiation dose (Dmax) that could guarantee parchment documents′ decontamination treatment, without significant alteration of their physical properties. Parchment samples were exposed to gamma radiation doses ranging from 10 to 30 kGy. The texture and colour of samples were assessed before and after the irradiation procedure, using a texture analyser and an electronic colorimeter. Hardness and springiness were determined based on texture spectra. Lightness (L?), Chroma (C), greenness vs. redness (a*) and yellowness vs. blueness (b*) values were obtained from colorimetric measures. Results indicate no significant effects of gamma radiation on the texture and colour of parchment for the studied doses.  相似文献   
43.
The Copaifera species (Leguminoseae) are popularly known as ‘copaíba’ or ‘copaíva’ and are grown in the states of Amazonas, Pará and Ceará in northern Brazil. The oleoresins obtained from these species have been extensively used owing to their pharmacological potential and their application in cosmetic and pharmaceutical preparations. In the present study, the development and validation of a novel, rapid and efficient RP‐HPLC methodology for the analysis of the diterpene (?)‐copalic acid (CA), pointed out as the only chemical marker of the Copaifera genus, are described. The regression equation (Y = 26,707x ? 29,498) was obtained with good linearity (r2 = 0.9993) and the limits of quantification and detection were 9.182 and 3.032 µg/mL, respectively. The precision and the accuracy of the method were adequate (lower than 4%). Finally, the validation parameters evaluated were satisfactorily met, so the developed method represents a suitable tool for application in the quality control of such natural products. Further studies aiming to develop analytical methodologies for each Copaifera species using a more representative number of chemical markers should be performed. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
44.
In recent years, there has been an increase of infectious diseases caused by different microorganisms and the development of antibiotic resistance. In this way, the search for new and efficient antibacterial materials is imperative. The main polysaccharides currently used in the biomedical and pharmaceutical domains are chitin and its derivative chitosan (CH) and alginates (ALG). In this study, a simple technique of Layer by Layer (LbL) of applying polycation CH and polyanion ALG was used to prepare CH/ALG multilayers on cotton samples via the electrostatic assembly with success. The CH/ALG cotton samples (functionalized) were investigated for their antibacterial properties towards Staphylococcus aureus and Klebsiella pneumonia using the international standard method JIS L 1902:2002. The antibacterial activity of the functionalized samples was tested in terms of bacteriostatic and bactericidal activity, and results showed that the samples exhibited a bacteriostatic effect on the two bacteria tested, as expected. In addition, samples with five layers (CH/ALG/CH/ALG/CH) were more effective in inhibiting bacterial growth. This new coating for cellulosic fibers is a new strategy and may open new avenues for the development of antimicrobial polymers with potential application in health‐care field. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
45.
In buildings, windows have a major influence on space heating demand and indoor environment both with respect to climate and daylight. To reduce the window coefficient of the overall heat transmission, we use aerogel. Aerogels have a high surface area, low density, open pore structure, and excellent insulation properties. We mixed pressure sensitive adhesive and aerogel (10, 15, and 20 mass%) using a homogenizer. A mixture of the adhesives and silica aerogels attached film can reduce thermal conductivity. Silica aerogels are characterized by a surface area analyzer (BET), a Fourier transform infrared spectrometer, a thermogravimetry (TG) analyzer, and probe tack method. Thermal conductivity was measured by a TCi thermal conductivity analyzer.  相似文献   
46.
Cellulosic biomass is a promising alternative energy resource from the viewpoint of sustainability. The use of waste materials as cellulosic biomass could additionally contribute to a recycling society. It is thus essential to develop safer processes in order to expand utilization of cellulosic biomass as a useful resource in the future. For example, in some cases, construction wastes contain wood preservatives, including metal oxides that can act as catalysts for the oxidation of organic materials. Copper(II) oxide (CuO) is a major component in wood preservatives and is known to catalyze the oxidation of cellulose. There is, therefore, possibility for spontaneous ignition within large piles of wood chips from construction wastes. In this study, we focused on the thermal behavior of a cellulose/CuO mixture, measured using a Calvet-type heat flux calorimeter. In addition, Fourier transform infrared spectroscopy and gas chromatography were applied to analyze the oxidative decomposition gases of the cellulose/CuO mixture, and a reaction mechanism was proposed. It was revealed that CuO promotes the oxidative decomposition of cellulose and increases the quantity of the gases that evolved from cellulose with a catalytic cycle. The influence of CuO on oxidation of cellulose is greater at lower temperatures and spontaneous ignition, fires, and explosions are likely to increase when wood chips containing CuO are stored for long periods of time.  相似文献   
47.
The aim of this study was to introduce a non-formaldehyde inorganic–organic hybrid sol–gel flame-retardant precursor (SiOP) containing phosphorous, nitrogen, and silicon and to compare its functional properties with those of the conventional formaldehyde-containing organic flame-retardant agent, organophosphonate (OP). SiOP was used at concentrations of 2%, 4%, and 8%, and OP was used at a concentration of 200 g/dm3. Both agents were applied to 100% cotton (CO) woven fabric by the pad-dry-cure method under the appropriate conditions. The presence of the SiOP and OP coatings on the CO fabric was confirmed by scanning electron microscopy, energy dispersive X-ray spectroscopy and Fourier-transform infrared spectroscopy. The results of the vertical tests of flammability and the thermogravimetric analyses showed that the presence of the SiOP coating changed the thermal degradation pathway of the CO fabric and resulted in an increase in the thermo-oxidative stability of the cellulose fibres. The thermo-oxidative stability was enhanced by the addition of higher amount of dry solids. At comparable dry solids contents, OP preserved significantly greater flame retardancy and thermo-oxidative stability than did SiOP. These results indicated that the SiOP precursor could not act as an effective alternative to the OP agent in the flame-retardant protection of CO fabric.  相似文献   
48.
49.
Malva sylvestris is a species used worldwide as an alternative to anti‐inflammatory therapies; however, its mechanism of action remains unknown. In this paper, the anti‐inflammatory effects of M. sylvestris alcoholic extracts were evaluated by measuring the pro‐inflammatory mediators PGE2 and PGD2 in desferrioxamine‐stimulated phorbol 12‐myristate 13‐acetate‐differentiated U937 cells. An HPLC‐DAD fingerprint of the M. sylvestris extract was performed and caffeic acid, ferulic acid and scopoletin were identified and quantified. An HPLC‐MS/MS method was developed and validated to separate and measure the prostaglandins. The lower limits of detection (~0.5 ng/mL for PGE2 and PGD2) and quantification (1.0 ng/mL for PGE2 and PGD2) indicated that the method is highly sensitive. The calibration curves showed excellent coefficients of correlation (r > 0.99) over the range of 1.0–500.0 ng/mL, and at different levels, the accuracy ranged from 96.4 to 106.4% with an RSD < 10.0% for the precision study. This method was successfully applied using U937‐d cells. A significant dose‐dependent reduction of PGE2 and PGD2 levels occurred using 10 µg/mL (10.74 ± 2.86 and 9.60 ± 6.89%) and 50 µg/mL of extract (48.37 ± 3.24 and 53.06 ± 6.15%), suggesting that the anti‐inflammatory mechanisms evoked by M. sylvestris may be related to modulation of these mediators. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
50.
In this work, carbon electrodes modified with aminophenols were developed for the production of pesticides biosensors based on acetylcholinesterase. The polymers were potentiodynamically deposited on a graphite electrode surface by the oxidation of monomers, 2-aminophenol, 3-aminophenol and 4-aminophenol. The electrochemical behaviour and surface analysis of the electrodes modified by polyaminophenols non-immobilized and immobilized on acetylcholinesterase were studied by cyclic voltammetry, electrochemical impedance spectroscopy and atomic force microscopy. Roughness values obtained for graphite electrodes modified with poly(4-aminophenol) and poly(4-aminophenol)/acetylcholinesterase were 174 and 86 nm, respectively. The acetylcholinesterase enzyme was immobilized on a graphite and a graphite modified with poly(4-aminophenol), and these electrodes were coupled in the flow system. Potentiometric response due to hydrogen ions generated by an enzymatic system in the presence of acetylcholine chloride substrate was evaluated. The results showed that the graphite/poly(4-aminhophenol) sensor presents high sensitivity to hydrogen ions when compared with other graphite/polyaminophenols sensors. The biosensor coupled in a continuous flow system was employed for the detection of dichlorvos. The detection and quantification limits were 0.8 and 2.4 μmol L−1 dichlorvos, respectively. This sensor reveals an efficient and promising material for biomolecules immobilization.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号